Fluid catalytic cracking (FCC) of petroleum fractions is a well-established refinery operation. In FCC, heavy hydrocarbon fractions (greater than about 20 to about 30 carbon atoms in length) are chemically broken down into lighter hydrocarbon fractions (less than about 12 to about 15 carbons in length), such as gasoline. The FCC unit usually comprises a reactor section connected to a regenerator section by standpipes. The catalyst itself is a finely divided solid and behaves like a fluid in the reactor, regenerator, and connecting standpipes, hence the designation "fluid" catalyst.
In the operation of the FCC process, fresh hydrocarbon feed, which may be preheated, is mixed with catalyst and undergoes cracking within the catalytic conversion zone of the reactor section. The catalytic conversion zone, in modern FCC units, is primarily located in the riser of the reactor section. For catalytic cracking to occur, the hydrocarbon feed (e.g., oil), must be vaporized to permit the hydrocarbon feed to diffuse into the pores of the catalyst (generally a zeolite) to the cracking sites.
The catalytic cracking reaction results in coke being deposited onto the catalyst to form "coked" or "spent" catalyst. Products exit the reactor in the vapor phase and pass to at least one main fractionator or distillation column for separation into desired fractions. The spent catalyst passes continuously from the reactor to the regenerator via a spent catalyst standpipe. In the regenerator, the coke is converted in an exothermic reaction to gaseous products by contact with an oxygen-containing gas. The flue gas passes out of the regenerator through various heat recovery means, and the hot regenerated catalyst is recirculated to the reactor via a return catalyst standpipe where it is again picked up by fresh hydrocarbon feed. Typically, heat released in the regenerator is carried to the reactor by the hot regenerated catalyst to supply heat for the endothermic cracking reactions. Typical fluid catalyst cracking systems are disclosed in U.S. Pat. Nos. 3,206,393 to Pohlenz and 3,261,777 to Iscol, et al., which are hereby incorporated by reference in their entireties.
Nozzles are used to inject the hydrocarbon feed, typically in the form of a liquid spray, into the catalytic conversion zone of the riser. To form a spray, typically, the hydrocarbon feed is combined with a dispersion medium, such as steam, to form a dispersed hydrocarbon stream. The one or more nozzles used to inject the dispersed hydrocarbon stream into the catalytic conversion zone may be axially or radially disposed. With axial nozzles, coverage is achieved by using one or more nozzles that extend into the riser section of an FCC reactor and terminate at a set of points within the riser cross sectional area. The axial nozzles are preferably oriented substantially vertical (preferably within about 10.degree. of the vertical axis of the riser) to create a flow of hydrocarbon feed that is preferably parallel to the upflowing catalyst. With radial nozzles, coverage is achieved by using a plurality of nozzles that are mounted around the perimeter of the riser wall. Preferably, the radial nozzles extend minimally into the riser itself. This orientation of the nozzles creates a flow of hydrocarbon feed that crosses with the upward flow of catalyst. Radial nozzles are preferably oriented with respect to the vertical axis of the riser at any angle from about 10.degree. pointing upward to about 90.degree.horizontal). To provide optimal catalytic cracking conditions, one or more nozzles in either orientation preferably collectively spray the dispersed hydrocarbon stream in a pattern that expands to cover the entire cross-sectional area through which the cracking catalyst is flowing. Improved coverage provides better catalyst-hydrocarbon feed mixing which enhances catalytic cracking reactions and minimizes thermal cracking reactions. Thermal cracking reactions produce undesirable products such as methane and ethane and decreased yields of more valuable FCC products.
In addition to full spray coverage, the nozzles preferably produce fine hydrocarbon feed droplets, preferably less than about 100 microns (.mu.m) in Sauter mean diameter (i.e., the diameter of a sphere having the same volume to surface area ratio as the measured droplets), as this is comparable to the size of individual catalyst particles. As droplet size decreases, the ratio of hydrocarbon feed drop surface area to volume increases, which accelerates heat transfer from the catalyst to the hydrocarbon feed and shortens hydrocarbon feed vaporization time. Quicker vaporization improves yield of catalytic cracking reaction products since the hydrocarbon feed as a vapor is able to diffuse into the pores of the catalyst. Conversely, any delay in hydrocarbon feed vaporization, and mixing of the hydrocarbon feed and catalyst, leads to increased yields of thermal cracking products and coke.
A variety of nozzles have been used for providing an atomized hydrocarbon stream into the catalytic cracking zone. FIG. 1 illustrates a prior art nozzle 1 having a bayonet 2 and a simple orifice tip 3. In this type of nozzle the hydrocarbon feed and dispersion medium are mixed upstream of the bayonet to form a dispersed hydrocarbon stream 4 that flows through the orifice 5. The orifice tip 3 is a plate positioned approximately two inches upstream from the bayonet outlet 6 with the diameter of the orifice 5 being smaller than the nozzle inner diameter 7. The orifice 5 provides a shear edge to break up the liquid and to spread the spray more widely across the reaction zone.
U.S. Pat. No. 4,640,463 to Krambeck et al. (hereinafter referred to as "Krambeck") discloses another type of nozzle useful for injecting a liquid hydrocarbon feed and dispersion gas into a catalytic cracking zone. The Krambeck nozzle is an example of a class of nozzles known as plain-jet atomizers. In plain-jet atomizers, liquid and gas are discharged together through a high-shear restriction. The Krambeck nozzle contains an inner conduit capped with a flow restriction device that is concentrically aligned with an outer conduit also capped with a flow restriction device. The nozzle operates by introducing hydrocarbon feed in the inner conduit and a dispersion gas such as steam in an annular space between the inner and outer conduits. The outer conduit extends longitudinally beyond the inner conduit so that mixing of the hydrocarbon feed and steam can occur between where the inner and outer conduits end. The dispersed hydrocarbon stream is atomized into the catalytic conversion zone through a flow restriction device located at the end of the outer conduit.
Another class of nozzles, known as "prefilming" nozzles, operate by producing a thin sheet of liquid that is contacted with a high-velocity gas, in one or more locations around the perimeter of the liquid sheet, in a space free of minute flow restrictions. The high velocity gas destabilizes the liquid sheet and breaks it into liquid droplets. Examples of this type of nozzle are disclosed in, for example, "Airblast Atomization" by Arthur H. Lefebvre, Prog. Energy Combust. Sci., Vol. 6, pages 233-261, (1980). Most prefilming nozzles require gas-to-liquid mass ratios of at least 2.0, and preferably 4.0, for effective atomization. In typical FCC units, the practical maximum of gas-to-liquid mass ratio is lower; preferably less than 0.10, and more preferably from about 0.03 to about 0.05.
In order to provide finer droplets with a fixed gas-to-liquid ratio, a plain-jet atomizer requires a smaller diameter orifice and a prefilming nozzle requires a thinner opening for the liquid film. However, smaller openings generally lead to greater feed-side pressure drop. With present-day refinery economics dictating that FCC units operate at feed rates far exceeding design, feed-side pressure drop is typically a very scarce resource. Often feed-side pressure drop constrains incremental throughput, thereby limiting profitability. In order to increase feed rate with minimal pressure drop penalty, some FCC practitioners tolerate as feed nozzles simple straight pipes with no tip device, and are thus unable to obtain the yield benefits of smaller drop size and better riser coverage. Other refiners turn down dispersion steam to make room for more hydrocarbon feed, which decreases pressure drop by reducing not only steam flow but also the amount of hydrocarbon feed which vaporizes, the latter being a larger impact. Unfortunately steam reduction also compromises both atomization and spray coverage because nozzle velocity decreases and the energy available to shear the hydrocarbon feed is smaller.
It is desirable to provide a nozzle for atomizing liquid in the presence of a dispersion medium that does not require a significant feed side pressure at a given feed rate and nozzle diameter to produce fine liquid droplets.